I. Field of the Invention
The invention relates to a circuit arrangement for the controlled supply of a load, as set forth in the classifying portion of claim 1.
II. Description of the Prior Art
For many electrical loads, for example portable devices, which include electric dry razors, electronic flash devices, fluorescent lamps and the like, it is desirable to provide a power supply circuit which can be operated without change-over switching from virtually all d.c. and a.c. mains anywhere in the world, and which supplies power to the load in the required manner, for example at a constant current or a constant voltage or a combination of both. If the load includes a battery or accumulator, so that for example in the case of a dry razor, the device can also be operated independently of the main supply, the power supply circuit should in turn be capable of charging up the battery, independently of the main supply voltage and frequency, or should be capable of feeding power to the load alone, for example when the battery is discharged.
In connection with the above-described situations of use of such a circuit arrangement, a further condition which gives rise to complications is that, because of the restricted amount of space, for example in a dry razor, the space required for accommodating a supply circuit arrangement can only be very small and at the same time the power loss must be kept at a particularly low level because the cooling capabilities are greatly restricted when dealing with a small volume. Finally, there is a requirement in respect of many portable devices for the device to operate satisfactorily, even under high ambient temperature conditions, for example on travelling to tropical countries.
In a known circuit arrangement (U.S. Pat. No. 4,005,351), the input voltage has a strong influence on the mode of operation because the speed of rise of the primary current depends on the input voltage. The period of time for which the first semiconductor switch in the form of a transistor is switched on is then reduced, with the same switch-off current, as the input voltage rises, and the resulting change in frequency leads to a corresponding change in the mean output power. Although, for the purposes of maintaining the output voltage at a constant value, the known circuit arrangement provides for influencing the primary current switch-off value and thus the period of time for which the switch is switched on, by applying a further voltage which is obtained during the non-conducting phase and which is applied to the control electrode of the second semiconductor switch, nonetheless that arrangement does not overcome the basic dependency on the input voltage. As the two control parameters, namely the primary current and the output voltage, are produced at different times, during the conduction and the non-conduction or blocking phases respectively, it is necessary to provide for storage of one value by means of a capacitor. This results in a reduction in the speed of control, and the circuit arrangement cannot therefore adjust immediately to fluctuations in the operating conditions.
A further circuit arrangement is also known [DOS (German laid-open application) No. 27 51 578], for producing a controlled voltage, using a blocking converter or transformer whose primary winding is connected to the input voltage source by way of a transistor which can be switched on by positive feedback, and an emitter resistor. As in the case of the circuit arrangement described hereinbefore, this circuit arrangement also has a severe dependency on the input voltage, which is due to the different speed of rise of the primary current, in dependence on the input voltage. In this case also, control in respect of the output voltage is achieved by producing a voltage during the non-conduction or blocking phase so that a storage capacitor is again required.
A feedback-controlled single-cycle blocking converter is also known [DOS (German laid-open application) No. 25 43 371], in which the associated transformer is respectively put into a condition of saturation, and then produces the switching-off effect. Besides the primary winding, the converter arrangement also has a secondary winding which is used at the same time for deriving the feedback voltage. That also applies in regard to a known push-pull d.c. voltage transformer [DAS (German published application) No. 12 30 119], in which control of the primary switches is effected by means of a voltage which is derived from the secondary winding.
In another known circuit arrangement (British published specification No. 20 00 394A) for supplying a d.c. motor and charging a battery or accumulator, the circuit arrangement, for example for an electric razor, the arrangement uses a blocking converter which is connected to the respective main voltage by way of a bridge rectifier and which supplies at the secondary side, depending on the condition of operation, the charging current for the accumulator or the operating current for the motor. The control action, for compensating for the varying input voltages, is produced by means of a complicated control circuit in the form of an integrated circuit which supplies switching pulses to the primary-side switching transistor of the blocking converter, the length of said pulses depending on the input voltage and the respective mode of operation of the arrangement. However, the known circuit arrangement does not fulfill all expectations. The cost thereof is comparatively high and the amount of space that it occupies is frequently excessively large. The integrated circuit requires its own supply voltage and accordingly a starting circuit for the converter. It also consumes power. When operating the motor from the mains, it is not possible to achieve or maintain a full charge in the battery.
A transistor converter circuit is also known [DOS (German laid-open application) No. 20 14 377], by means of which on the one hand a charging current for charging a battery and on the other hand a higher direct current for driving a motor can be produced from a main a.c. voltage. For this purpose, the arrangement has a through-flow converter which is operated at high frequency and has a saturable core, being connected at the primary side to the rectified main voltage and supplying the desired currents at the secondary side. The known circuit can only be operated on a given main voltage, and therefore does not automatically adapt to different voltages. As the core of the transformer goes into a condition of saturation in each case, the level of efficiency is low, and problems arise in regard to heat.
For operating a motor-driven electric razor over a wide range of a.c. input voltages, a circuit is known (U.S. Pat. No. 4,001,668), in which a capacitor is charged up by way of transistors to a predetermined value of about 100 V. This circuit is not suitable for producing higher currents because there is no transformation action. It is also only suitable for loads which have a comparatively high operating voltage. With lower levels of operating voltage, the degree of efficiency deteriorates seriously. The amount of space occupied by the arrangement is comparatively large.
Finally, a charging device for accumulators or batteries is also known (U.S. Pat. No. 3,943,423), which can be used, without switching over, for different input voltages. A transistor switch is disposed in a feedback circuit and represents a variable resistor which takes over the current control function. There is no transformation effect in this arrangement, so that higher currents cannot be produced.